I feel the same way about signing Mookie that I do about the Sox chances of getting a WC berth this year. Possible, but highly unlikely.
Fenway needs two outstanding outfielders in order to be successful and Beni doesn't fill the bill. So what we're talking about if we lose both of them is finding two outstanding fielders, one of CF and the other for RF, before the 2021 season. Good luck with that.
IMO we'll do well to find one outfielder to come close to replacing Mookie's offense and being average defensively - which is a definite downgrade. Consequently we're going to need someone out there to cover some of the ground that Mookie had covered in the past few years.
So-and-so and his two brothers and six cousins may be available but that doesn't mean they're going to be signed with the Red Sox. Is that a gamble we want to take....to have three outfielders with Benni's defensive skills patrolling the outfield in Fenway, even if it saves...say... $5MM?
OTOH JBJ's status is guaranteed if the FO wants him. One in the hand being worth two in the bush DD may (should) find it more palatable to overpay for him until/unless the situation with Mookie is resolved.
If they can sign Mookie in the off season then JBJ could be made available. If they can't sign Mookie they need to prepare for life without him, and having JBJ in CF is a good start.
Everyone wants to rail on about JBJ but a GG CF'er who's batting .224/.728 and hitting 9th isn't the reason this team probably isn't going to the playoffs.